[H-GM02] Geomorphology

[HGM02-09] Regolith evolution on soil-mantled hillslopes in bedrock-dominated landscapes: A weathering profile perspective from the eastern margin of Tibetan Plateau

*Wei Wei^1,2, Yuki Matsushi¹, Hiroyuki Matsuzaki³ (1.Disaster Prevention Research Institute, Kyoto University, 2.Faulty of Geosciences and Environmental Engineering, Southwest Jiaotong University, 3.Micro Analysis Laboratory, Tandem Accelerator, The University of Tokyo)

Keywords:Regolith, Denudation, Cosmogenic 10Be, Rate limitation, Tectonic forcing

Regolith, referring to soil and saprolite weathered and converted from bedrock, has an intimate relationship to landscape shaping on the Earth’s surface because most geomorphic processes are only capable of transporting unconsolidated materials such as mobile regolith. In terms of regolith-mantled landscapes with both soil and saprolite existing, ‘soil’ is the physically mobile material produced by the underlying saprolite, and ‘saprolite’ is the immobile and deep weathering mantle produced by the underlying bedrock. Therefore, quantifying relevant parameters including regolith production and denudation rates is essential to understand the evolution of landscapes over geologic timescales and the response of landscapes to external forcing. Here by definition, regolith production rate corresponds to the conversion of bedrock to regolith, and regolith denudation rate reflects the removal of regolith by physical erosion and chemical weathering.
To address the quantification of these rates, the revolutionary use of terrestrial cosmogenic nuclides measured by accelerator mass spectrometry (AMS) has opened a door in recent decades. For example, cosmogenic ¹⁰Be inventory from depth profile and/or catchment sediment has enabled the estimation of denudation/erosion and soil production rate of Earth surface. When combining the ¹⁰Be analysis with geochemical mass balance calculation using immobile elements (e.g., Zr) within a weathering profile, one can explicitly extract the respective proportion of chemical weathering and physical erosion from a certain denudation rate.
The Longmenshan area is located in the eastern margin of Tibetan Plateau and is the transition zone of the Tibetan Plateau and Sichuan Basin, where extremely high-relief and bedrock-dominated landscapes controlled by strong tectonic uplif are present. Pertinent studies in this region have mainly focused on the catchment-wide erosion rates derived from cosmogenic ¹⁰Be measurements of stream sediments to address (1) tectonic damage on the erodibility of bedrock, (2) hillslope-channel adjustment to higher erosion rates, (3) surface erosion rates response to tectonic forcing and associated landscape evolution, and (4) dilution effect of ¹⁰Be in detrital quartz by earthquake-induced landslides. However, to our best knowledge, no attempts have been made to quantify the long-term rates of denudation (or soil production), physical erosion and chemical weathering via depth profiling approach because representative weathering profiles are rarely observed in such rugged topography. Here, we provide a rare dataset of in situ ¹⁰Be and geochemical compositions from a high-resolution depth profile in Wenchuan area to (1) determine the rates of denudation, soil production, weathering and erosion for the regolith-mantled hillslope, (2) characterize the rate-limited framework of kinetic limitation vs. transport limitation on the hillslope, (3) discuss any wider implications for regolith evolution against to tectonic forcing. This study may facilitate to better understand the dynamic coupling between chemical weathering and soil production under a rapid erosion regime, which may help explain how the soil-mantled landscapes transform to bedrock-dominated topography.